From HD 5770 to GTX 660ti or GTX 590?

[redness]

Hello.

Currently I'm using an HD 5770 was hoping I could get some advice on what I should upgrade to next for 2013(Gaming) .

Right now I'm thinking of going for the GTX 660 ti but I noticed in benchmarks that the GTX 590 is a lot faster and performs way better. Why is that? I'm not completely familiar with how video card trends work but isn't the latest model supposed to be the one that performs better? Is there something I'm not catching?

If I'm after performance should I go for the GTX 590 or is there something with it like an absurd power requirement or overheating issue that I'm not getting?

Thanks for any help and other video card recommendations that are in the $300 price range that would work well for 2013-2014 are welcome.

 

[Rockdpm]

Well how much wattage is your power supply that you have now?

 

[redness]

Embarrassingly I don't know.

Can we just assume that I would have enough for both cards? Also, why IS the 590 performing better than the more later models? Did the 590 come after the 680 or something?